1. Field of the Invention
The present invention relates in general to a step controller, and more particularly to a correcting element of a step controller that reduces a reset time of the step controller as a function of an input signal.
2. Description of the Related Art
A conventional step controller is disclosed by German Patent DE 40 04 183 A1. This step controller has a three-level switch with an operating threshold and with hysteresis. The step controller also has a negative feedback through a feedback element containing an integrator. To achieve PI (proportional-integral) action by the controller, an error signal, used as the input signal, is multiplied by a factor Kp and fed back directly as the proportional (P) component. The input signal is also fed back through a parallel branch containing an integrator to the three-level switch. In the integrator, the input signal is integrated only during the intervals between the output signals from the three-level switch, i.e., during the pauses of the output control pulses. Thus, the linear behavior of the P component across the control range is attained in an advantageous way.
The step controller has only a slight dependence of the effective controller parameter on the controller input signal. Therefore the control parameters of the step controller can be optimized like a linear controller so that the step controller can be used as a self-adapting controller. To adapt the controller to a process, for example, a step excitation can be applied to the process and the step response of the process can be evaluated. By evaluating the process response, a suitable process model and its parameters can be calculated, and a linear controller can be determined for the parameters of the process identified in that manner to give optimum yield. In the known step controller, furthermore, a special adaptation of the operating threshold of the three-level switch can be used to reduce the frequency of switching so as to minimize wear of the positioning element. In this adaptation, the duration of the control pulse increases with the magnitude of the excitation.
Although the conventional step controller is generally thought to provide acceptable control action, it is not without shortcomings. Namely, the conventional step controller still has residual, nonlinear behavior, and therefore the controller parameters established for the controller following parameter optimization differ from the controller parameters that are finally effective on operation of the step controller. That can lead to the transient response of the controller changing with the magnitude of the excitation. Furthermore, the number of control pulses increases with increasing excitation, and that in turn increases the wear of the positioning element again.
An object of the present invention is to provide a step controller in which the control action can be further improved.
The present step controller improves the approximation of the controller behavior to that of a linear controller by use of a correcting element which reduces the controller reset time as a function of the error signal used as the input signal, so that the reset time is shorter for a larger input signal.
The present invention is based on the recognition that the effective reset time for the known step controller increases with increasing excitation. Only the controller amplification was independent of the excitation. That is, for large error signals the step controller initially behaved as a P controller. The integral (I) component became active only with diminishing error signal. The greater the excitation was, then, the more slowly it was corrected. For a controller adjustment according to the optimum process and with small changes in the set point, there was still an overshoot in the control magnitude, which decreased for larger steps in the set point. If the controller were adjusted so that large changes in the set point could be adjusted aperiodically, then the control magnitude could still overshoot with small steps in the set point. In spite of the improvements made for the linearity of the P component in the known step controller, the controller was still always nonlinear with respect to the reset time. This effect is compensated by the use of the correcting element in the present step controller.
Practically ideal linear behavior of the step controller can be attained advantageously with the correcting element of the present invention. For example, when the step controller controls a linear process, the parameters can be set so that if a sudden excitation occurs, then the control magnitude is adjusted aperiodically to a new steady final value, independent of the magnitude of the sudden excitation.
For reliability, the corrected reset time is limited to a minimum value, preferably five times the sampling time. Limitation to one-tenth of the reset time Tn is an acceptable alternative.
If the operating threshold of the three-level switch is adjusted only as a function of the proportional component of the controller, then the step controller is also advantageously well suited as a controller with proportional-integral (PI) behavior in which the reset time is very large so that the I component is practically negligible.
When the set point is constant, the operating threshold can be set advantageously to half of the magnitude of the proportional component, for example. If the set point changes, the operating threshold can be set to one twentieth of the magnitude of the proportional component, for example. That gives a good compromise between the number of switching processes and the control quality. In addition, ramp changes in set point can be followed better with such a controller.
The above and other features of the invention including various and novel details of construction will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular step controller embodying the invention is shown by way of illustration only and not as a limitation of the invention. The principles and features of this invention may be employed in varied and numerous embodiments without departing from the scope of the invention.